1. A cross section of the English population showed an increased prevalence of hypothyroidism in regions with high water fluoridation compared to areas of low fluoridation.
2. Two demographically comparable regions differing in fluoridation by provider showed increased hypothyroidism prevalence in the artificially fluoridated region (levels >0.3mg/L).
Evidence Rating Level: 3 (Average)
Study Rundown: While fluoridation of drinking water has been an important public health measure to reduce dental caries, concerns about adverse effects of artificial water fluoridation have started to come to light. In particular, pre-clinical and epidemiological studies have suggested that excess fluoride may lead to a greater risk of hypothyroidism. In this cross-sectional analysis of the English population from the Quality Outcomes Framework prevalence data base, the authors found an increase in hypothyroidism prevalence of about 50% in the areas with most fluoridation compared to areas of lower fluoridation. This association remained robust even after controlling for other risk factors predicting hypothyroidism. As further evidence of this association, one region with high fluoridation had a 50% higher prevalence of hypothyroidism when compared to a demographically similar area with low fluoridation. After adjusting for other known hypothyroidism risk factors, the risk was estimated to be double in the highly fluoridated region.
While this study does not follow a cohort of patients over time, the epidemiological data strongly supports a significant effect of fluoride on hypothyroidism pathogenesis. One particular strength of this study is the high inclusion rate (98.9%) and high rate of annual general practitioner visits by the English population (80-89%). However, one drawback that reduces the generalizability of this study is that the absolute rates of hypothyroidism were not reported for each study group. Further, while this makes a strong epidemiological case for the association, a biochemical mechanism has yet to be elucidated. Overall, this study makes a strong argument for beginning to re-evaluate community fluoridation practices as a public health measure.
Relevant Reading: Fluoride in Drinking Water: A Scientific Review of EPA’s Standards
In-Depth [cross-sectional study]: 7935 out of 8020 (98.9%) general practitioner practices were included from the Quality Outcomes Framework database, which reported prevalence of hypothyroidism by practice. These practices were then associated with distinct water supply zones and each zone was assigned an average and maximum water fluoridation level measured by the Drinking Water Inspectorate. The lower two-thirds of fluoridation levels (≤0.7 mg/L) were grouped and compared against the upper third of fluoridation levels (>0.7mg/L), and binary logistic regression models were used to correlate fluoridation and hypothyroidism prevalence. Predictors of increased hypothyroidism risk, such as proportion of women registered, proportion of patients over 40 years of age, and Index of Multiple Deprivation (IMD) were included in the models as covariates. Both average and maximum fluoridation levels showed an association with hypothyroidism prevalence. The upper third of fluoridated regions (max fluoride >0.7 mg/L) had an unadjusted OR for hypothyroidism of 1.452 (CI95 1.268-1.662) and an adjusted OR of 1.621 (CI951.379 to 1.904). In a separate analysis, two demographically comparable regions, Greater Manchester and West Midlands, had their hypothyroidism prevalence rates compared as West Midlands has their water source artificially fluoridated (>0.3 mg/L) while Greater Manchester does not (≤0.3mg/L). The sample from West Midlands also had an increased unadjusted OR of 1.536 (CI95 1.156-2.041) and adjusted OR of 1.935 (CI95 1.388-2.699) for hypothyroidism.
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